In the prior art digital subtraction angiography, abbreviated to DSA, is already a widely known image recording method for recording images of a vascular system of a patient with high quality, showing the vessels clearly. On the one hand images of the vascular system are recorded, with a contrast agent having been administered to the patient beforehand, so that the contrast agent-filled vessels can be identified particularly clearly on said images. When mask images, which were recorded before the contrast agent was present in the vascular system to be recorded without moving the patient, are subtracted from these contrast agent images (frequently also referred to as fill images), apart from noise effects only the signal components of the contrast agent remain, so that an excellent assessment of the resulting DSA images is possible.
DSA is not only used in instances where the vascular structure of a patient is essentially mapped or assessed but for moving images, in other words for example when the movement of the heart is to be observed. Time-based two-dimensional digital subtraction angiography is primarily known here, in which in the fill phase, in other words when the contrast agent is essentially evenly distributed in the vascular system being examined, two-dimensional fluoroscopy images of the patient are recorded repeatedly, from which for example mask images can be subtracted according to the cardiac phase. However it has also been proposed that a four-dimensional angiography data record should also be created when recording the vascular system in the region of the heart, to show the movement of the heart and the surrounding vessels over an entire cardiac cycle, in other words a three-dimensional volume moving over a cardiac cycle. Angiography data records showing this are referred to as four-dimensional angiography data records, as they show a temporal profile of a three-dimensional volume.
To record such four-dimensional angiography data records, it has been proposed, when an x-ray facility with a C-arm is used, for example in an angiography laboratory, that projection images should be recorded from different projection directions at different time points of the cardiac cycle. If an electrocardiogram (EKG) is traced at the same time, the recorded projection images can be assigned to different phases or time segments of the cardiac cycle. In other words the cardiac cycle is broken down into different time segments, with the projection images recorded within a time segment being organized respectively into a set of projection images. Each of these sets of projection images is then used respectively to generate a three-dimensional reconstruction image data record assigned to the time segment. If these three-dimensional reconstruction image data records are then combined by temporal assignment in respect of the cardiac cycle, the four-dimensional angiography data record results. Ultimately the known procedure for x-ray facilities with a C-arm is a multisegment reconstruction based on a retrospective evaluation of the EKG signal, which was recorded at the same time as the projection images.
However this approach has disadvantages. Firstly it requires a number of rotations of the C-arm. This is because the irregularity of the heart movement means that the projection images of a time segment are not necessarily evenly distributed over the covered projection directions. An attempt is therefore made to achieve the broadest possible data base, which allows halfway even coverage of the projection angle interval to be recorded for all time segments. However this is not always possible because of the unpredictability of the heart movement. This unpredictable heart movement during the different cardiac cycles is also the reason why the retrospective evaluation of the EKG signal is not always useful in practice. It should in particular be noted here that the acquisition time is generally very long, for example 20-30 seconds. There is a high level of radiation exposure and the timing in respect of the contrast agent bolus is extremely complex, particularly when a number of C-arm runs are required and contrast agent may even have to be administered more than once.